Northern Kalahari groundwaters: Hydrologic, istopic and chemical studies at Orapa, Botswana
Abstract
Most students of the Kalahari thirstland have been in agreement that any groundwater encountered should be fossil as no rain recharge could occur in the vast sand-covered regions at present. This conclusion is challenged in the present study during which intensive observations were made over three years on 27 boreholes and several shallow wells, situated in the sand-covered area of Orapa, in the northern Kalahari. Three aquifers are of importance in the region: a phreatic aquifer in the sandy Kalahari beds; a heterogenous aquifer in the underlying highly jointed basalt and a confined aquifer in the deeper-lying Cave sandstone. Rest level observations suggest that the phreatic Kalahari beds aquifer and the confined Cave sandstone aquifer are interconnected by occasional joints and solution zones in the intermediate basalt. The chemical composition of the various waters is in good agreement with the hydrological model. Tritium and 14C measurements show clearly that the phreatic aquifer is constantly recharged by rain water. The confined Cave sandstone aquifer, on the other hand, is devoid of tritium and devoid of, or low in 14C, indicating that this aquifer is not leaking, or leaking very slowly and water exchange with the upper aquifers is negligible. It is felt that intensive abstractions from this aquifer may, however, provide space for active recharge. Stable-isotope determinations rule out any recharge from pans, lakes or the river system in the north. Recharge occurs by direct rain infiltration, without any intermediate evaporation.
- Publication:
-
Journal of Hydrology
- Pub Date:
- August 1977
- DOI:
- 10.1016/0022-1694(77)90132-9
- Bibcode:
- 1977JHyd...34..203M